. Laboratory work in bacteriology. Bacteriology. 454 BACTEBIOI^GY. where the air is to be examined; the outer cap is removed and the watei: in the aspirating bottle is then allowed to escape. Any desired voIt ume of air can thus be drawn through the tube. The bacteria pres- ent in the air, coming into contact with the gelatin, become fixed and after the tube is set aside they develop. The colonies can then be counted, and, inasmuch as the volume of air drawn through the apr paratus is known, the average number of germs per liter of air can be ascertained. A more exact method, requiring however
. Laboratory work in bacteriology. Bacteriology. 454 BACTEBIOI^GY. where the air is to be examined; the outer cap is removed and the watei: in the aspirating bottle is then allowed to escape. Any desired voIt ume of air can thus be drawn through the tube. The bacteria pres- ent in the air, coming into contact with the gelatin, become fixed and after the tube is set aside they develop. The colonies can then be counted, and, inasmuch as the volume of air drawn through the apr paratus is known, the average number of germs per liter of air can be ascertained. A more exact method, requiring however very expen- sive apparatus, is that of Petri. In this method the air is drawn through a sterile sand filter Fig. 66 d, p. 469; The suspended particles, dust and organisms,- are held back by the well-packed, fine grained sand. The contents of the filter are then distributed into a num- ber of Petri dishes, gelatin is added and after thorough mixing it is al- lowed to solidify. The colonies that now develop can be examined and counted with the same^ ease as in water analysis. In this method it is possible for a large number of bac- teria to adhere to a single grain of sand and when development takes place the result will be but one colony. The objection can be over- A ^ come, in part, by substituting a sol- natiM of ;*"-s°ed^™k- ublfe compound for the sand. Thus, Tucker's; 5—Straus-Wurtz's. , , ,. , , . n powdered sodium sulphate, or better, cane-sugar may be employed. Pilfers containing sugar have been employed by Miquel, and by Sedgwick: and Tucker (Pig. 60 a). On the addition of gelatin, the s'ugar dissolves leaving the germs present in a fine state of division. The nuinber of colonies, thus obtained, repre: sents quite closely the number of cells originally present in the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations ma
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